Abstract High attrition rates - particularly at the late stage of drug development ? contribute to the estimated $2.0B cost of bringing a drug to the market. Furthermore, many of the drugs that reach the market benefit only a subset of patients. Currently, there is a lack of target-specific biomarkers that can better identify responsive patients. Consequently, the benefits from many mainstay treatments, including anti-angiogenic drugs, are still limited. This adds significant costs to the healthcare system by the unnecessary treatment of patients that are not likely to respond to a specific therapy. Therefore, the development of a non-invasive PET imaging-based companion diagnostic (CD) using a radio-labeled version of the therapeutic agent itself, will help stratify patients and improve disease outcomes. This is because the CD will i) provide quantitative measures of target expression in patient tumors at the time of treatment to select patients; and ii) enable the staging of therapeutic modulation, including increase/decrease of target prevalence during the treatment cycle; and iii) allow for timely switches to alternative treatment when imaging shows the failure of the ongoing therapy. We propose to test the feasibility of developing a [18 F] labeled cys-tagged vascular endothelial growth factor-3S (VEGF-3S) as a novel non-invasive PET-imaging based CD, to be used with VEGF-3S, a novel (and patented) VEGF antagonist generated through targeted disruption of 3 heparin sulfate proteoglycan (HSPG) binding residues in VEGF-A 165. The proposed CD will be based on VEGF-3S engineered with N-terminal 15- aa fusion tag containing cysteine residue (cVEGF-3S) for site-specific 18F radiolabeling. A positive [18F] cVEGF-3S PET signal from tumor sites in a patient will confirm the prevalence of VEGFRs targeted by VEGF- 3S only. Thus, VEGF-3S with the CD will have significant clinical advantages over the existing VEGF targeted drugs providing precision medicine to individual patients. We will establish the target specificity of [18F] cysVEGF-3S in this proposal which will be the quantitative milestone to progress to Phase II for IND enabling studies.